Vehicles that operate underwater are useful for performing tasks below the sea surface in such fields as deep water salvage, the underwater telecommunications industry, the offshore petroleum industry, offshore mining, and oceanographic research. (See, e.g., U.S. Pat. Nos. 3,099,316 and 4,502,407). One class of underwater vehicle is designated an autonomous underwater vehicle (AUV). AUVs are so named because they can operate without being physically connected to a support platform such as a land-based platform, an offshore platform, or a sea-going vessel.
Commonly used AUVs are essentially unmanned submarines that contain an on-board power supply, propulsion means, and a pre-programmed control system. In a typical operation, after being placed into a body of water from a surface platform, an AUV will carry out a pre-programmed mission, then automatically surface for recovery. A recovery boat is then dispatched to collect the surfaced AUV. The recovery procedure can be performed directly from the recovery boat or with the assistance of a diver. This procedure entails attaching a lift cable to the surfaced AUV so that it can be hauled out of the water using a crane or winch. Once recovered, the AUV is transferred to the surface platform or other servicing site where data obtained from the mission can be down-loaded, the AUV's batteries recharged, other components serviced, and new mission instructions programmed into the AUV's control device. The AUV is then redeployed into the body of water so that it can carry out another mission.
In this fashion, AUVs can perform subsurface tasks without requiring either constant attention from a technician or a physical link to a surface support platform. These attributes make AUV operations substantially less expensive than similar operations performed by underwater vehicles requiring a physical linkage to a surface support platform (e.g., remotely operated vehicles).
AUVs, however, suffer practical limitations rendering them less suited than other underwater vehicles for some operations. For example, because AUVs typically derive their power from an on-board power supply of limited capacity (e.g., a battery), tasks requiring a substantial amount of power such as cutting and drilling are not practically performed by AUVs. In addition, the amount of time that an AUV can operate underwater is limited by the capacity of the on-board power supply. Thus, AUVs must surface, be recovered, and be recharged between missions.
This recovery, servicing, and redeployment step reduces the productive operating time of an AUV. Moreover, it creates the additional expense associated with deployment of a recovery boat, diver, etc. In addition, the recovery and redeployment processes increase the likelihood that the AUV will be damaged. For example, AUVs can be damaged during surfacing by colliding with objects on the sea surface such as the surface support vessel. AUVs can also be damaged during the recovery process by colliding with the recovery cable, the side of a surface vessel or boat, or a portion of the crane or winch. In rough seas, recovery is hampered and made more dangerous by vertical heave, the up and down motion of an object produced by waves on the surface of a body of water. Severe vertical heave can render AUV recovery impractical.
Because AUVs are not physically linked to a surface vessel during underwater operations, communication between an AUV and a remotely-located operator (e.g., a technician aboard a surface vessel) is limited. For example, AUVs typically employ a conventional acoustic modem for communicating with a remotely-located operator. Such underwater acoustic communications do not convey data as rapidly or accurately as electrical wires or fiber optics. Transfer of data encoding real time video signals or real time instructions from a remotely-located operator is therefore inefficient. As such, AUVs are often not able to perform unanticipated tasks or jobs requiring a great deal of operator input without first being recovered, reprogrammed, and redeployed.